If your CB Radio wont transmit, then the rig itself is useless because you can't speak to your pals and that is the whole point of owning a CB radio. If you're CB won't transmit, below are solutions to get you transmitting back on airwaves:
1)If you have just replacement the microphone, make sure the pins inside the connector are wired accordingly to your radio, if they are not, it can cause your mic not to key. Cobra mics won't fit Harvard CB radios, as they are wired different, just remember that.
2)Make sure the wires inside the microphone and connector are not disconnected, if they are get a soldering iron and some solder wire and solder them back on. If the wires are disconnected from the microphone, it cannot function and will not key usually.
2)Make sure the microphone's key button is working, this is by unscrewing the microphone and when your press the mic button in, does the switch behind it click? If not the circuit is not being created, therefore the mic will not key and your CB radio will not transmit. So try a mate's microphone or buy a replacement of the internet.
4)Make sure the coax wire that you use to connect your CB radio to your antenna is not damaged. If it is, it can prevent your CB radio to transmit because the radio waves are not getting to the antenna exactly, as they will be leaking out into other peoples computer speakers! To tell if the coax you are using is bad, look for green copper oxide on the wire, if you see anything like that, it can be the cause this problem and will need replacing.
5)If you are using the CB radio in your house, the power supply unit can be faulty. So get a multimeter and measure the voltage it gives out, it should be in the range of 12 -13 volts, no less. If the voltage reading is not a minimum of 12 volts, your rig is not get the power it needs to dx, so it just won't transmit.
6)Check the output transistor inside your CB radio because it could be burnt out because of a high SWR reading.